The study is intended to explore the therapeutic usefulness of inactive precursors of drugs or pro-drugs, to be activated at specific sites by the action of organ specific enzyme systems. For example gamma-glutamyl derivatives of drugs are expected to be accumulated at the sites of localization of gamma-glutamyl transpeptidase where the active drug would be released from its inactive precursor and exert its activity. The effectiveness of this approach was demonstrated by the synthesis of yield-glutamyl DOPA (i.e., L-yield-glutamyl-L-3,4-dihydroxyphenylalanine). Administration of this derivative to small laboratory animals leads to its preferential accumulation in the kidney, release of DOPA and its conversion to dopamine by the action of kidney aromatic amino acid decarboxylase. The dopamine that is selectively generated in the kidney exerts a potent local activity, increasing renal plasma flow and urine output. Because of this organ-specific activity, systemic effects and untoward reactions do not occur. Exploratory studies are being carried out towards an extension of this principle to other pro-drugs.